1. Field of the Invention:
This invention relates generally to injection molds. More particularly, the invention relates to stack molds in which a number of mold plates are arranged between the press platens for simultaneously molding a number of parts.
2. Prior Art:
Injection molding has heretofore been based upon the theory that the part to be molded should be placed in the mold such that the main molding surfaces are in planes parallel to the platens of the press. This has been due to the belief that the clamping force of the press should be utilized to close the mold in the same plane or direction as the closing direction of the press. As a result, the area available for molding is limited to the area of the platens existing between the columns of the press.
An improvement over this limitation was accomplished with stack molds, in which two or more pairs or layers of mold plates are placed between the platens generally parallel thereto. While this arrangement increases the molding capacity of the presses, especially for flat parts, it is still restricted since injection devices must be used which penetrate the part being molded in order to reach all of the cavities to be filled. This eliminates use of such stack molds for a large number of parts that do not have openings permitting such penetration. Further, such systems require that the injection device be withdrawn to permit removal of the part. Moreover, there is a limit to the distance the molten material can be conveyed away from the nozzle of the press and still permit a thermally balanced distribution of the plastic into each cavity level. As a result of these limitations, very few stack molds have been successfully built with more than two levels of injection.
Prior art molds are further limited in their capacity because of the requirement for observing a limiting ratio between the surface of the part to be molded and the clamping force of the press. The projected surface of the part dictates the force required to keep the mold closed during the injection, regardless of how thin or light-weight the part may be.
Up to now, basically flat, thin-wall and relatively light-weight parts such as discs, lids, trays, partitions, hangers, plates, cutlery, frames, etc., have been produced in small quantities compared to the injection capacity of the press utilized. This normally results in high production costs and has therefore made injection molding an expensive process for a large number of parts which should otherwise be economical to manufacture. There is thus a need for molds capable of producing as great a number of parts as the injection press has the capacity to produce.